SEGMENTATION OF THE VITELLUS. 



537 



Fig. 194. 



mucous membrane of the Fallopian tubes, and envelopes the egg 

 in a layer of nutritious material. 



A very remarkable change now takes place in the impregnated egg, 

 which is known as the spontaneous division, or segmentation, of the 

 vitellus. A furrow first shows itself, 

 running round the globular mass of the 

 vitellus in a vertical direction, which 

 gradually deepens until it has divided 

 the vitellus into two separate halves or 

 hemispheres. (Fig. 19-i, a.) Almost at 

 the same time another furrow, run- 

 ning at right angles with the first, 

 penetrates also the substance of the 

 vitellus and cuts it in a transverse 

 direction. The vitellus is thus divided 

 into four equal portions (Fig. 194, b), 

 the edges and angles of which are 

 rounded off, and which are still con- 

 tained in the cavity of the vitelline 

 membrane. The spaces between 

 them and the internal surface of the 

 vitelline membrane are occupied by 

 a transparent fluid. 



The process thus commenced goes 

 on by a successive formation of fur- 

 rows and sections, in various direc- 

 tions. The four vitelline segments 

 already produced are thus subdivided 

 into sixteen, the sixteen into sixty- 

 four, and so on ; until the whole vi- 

 tellus is converted into a mulberry- 

 shaped mass, composed of minute, 

 nearly spherical bodies, which are 

 called the "vitelline spheres." (Fig. 

 194, c.) These vitelline spheres have 

 a somewhat firmer consistency than 

 the original substance of the vitellus ; 

 and this consistency appears to in- 

 crease, as they successively multiply in numbers and diminish in 

 size. At last they have become so abundant as to be closely 

 crowded together, compressed into polygonal forms, and flattened 



SEGMENTATION OF THE VITELLL-S. 



